In gas turbine engines, compressed air discharged from a compressor section and fuel introduced from an external source are mixed together and burned in a combustion section. The mixture is directed through a turbine section, where the mixture expands to provide rotation of a turbine rotor. The turbine rotor may be linked to an electric generator, wherein the rotation of the turbine rotor can be used to produce electricity in the generator.
Gas turbine engines are known to produce an exhaust stream containing a number of combustion products. Many of these byproducts of the combustion process are considered atmospheric pollutants, and increasingly stringent regulations have been imposed on the operation of gas turbine power plants in an effort to minimize the production of these gasses. Of particular concern is the regulation of the production of the various forms of nitrogen oxides collectively known as NOx. It is known that NOx emissions from a gas turbine increase significantly as the combustion temperature rises. One method of limiting the production of NOx is the use of a lean mixture of fuel and combustion air, i.e. a relatively low fuel-to-air ratio, thereby limiting the peak combustion temperature to a level below the threshold for NOx production. However, higher combustion temperatures are desirable to obtain higher efficiency and reduced production of carbon monoxide.
Two-stage combustion systems have been developed that provide efficient combustion and reduced NOx emissions. In a two-stage combustion system, the majority of the fuel and air enter the pre-mixed combustion stage to reduce NOx emissions. In pre-mixed combustion, the air and fuel are mixed together in a pre-mixer assembly that is upstream of a main combustion chamber of the engine. A small diffusion stage is included for obtaining ignition and low load flame stability. In diffusion combustion, the air and fuel are mixed together and ignited in the combustion chamber.
Gas turbine engines have been designed to combust a broad range of hydrocarbon fuels, such as natural gas, kerosene, biomass gas, etc, and more recently gas turbines engines have been designed to combust syngas produced from integrated gasification combined cycle applications. The syngas has a much higher flame speed than natural gas and is more susceptible to flame flashback when applied in pre-mixed combustion. Flame flashback in the pre-mixer assembly of gas turbine engines is undesirable, as it can cause damage to the components in and around the pre-mixer assembly, i.e., the flame may anchor onto the components and may burn through them.
Specifically, flame flashback may be caused when the turbulent burning velocity of the air and fuel mixture exceeds the axial flow velocity in the pre-mixer assembly, especially in low velocity regions near the boundary layer of the pre-mixer assembly. Flame flashback can also occur in recirculation zones that are caused by abrupt changes in the area of the flow path of the air and fuel mixture, such as at an aft end of a swirler assembly of the pre-mixer assembly, which provides an exit for the air and fuel mixture from the pre-mixer assembly into the combustion chamber.